1. Field of the Invention
The present invention relates to a rotary capping apparatus for applying a screw-on type cap to a filled container and, more particularly, to a rotary capping apparatus having an integrated feedback control system to precisely regulate the torque applied to such a container.
2. Description of Related Prior Art
Rotary capping devices are commonly used in industrial container filling operations such as pharmaceuticals wherein containers are filled with liquid or powder and then capped. In such filling operations empty containers are initially placed in so-called unscrambling devices, which are advanced to a filling line for filling, and then carried to the capping station via conveyor belts, starwheel devices and other apparatus for capping.
The screw-on type caps are disposed in unscrambling devices and then fed to the capping apparatus by way of conveyors and/or vibratory guides. Next, the caps are placed on the containers by a so-called pick-and-place mechanism. At the torquing station, the capping apparatus clamps the filled containers and grips the caps pre-positioned on the containers and rotates the caps onto the container. After a predetermined torque is applied by an adjustable chuck, the torquing operation is completed and the installed cap is released. The container clamping means is then released and the container is moved away from the capping apparatus by a suitable conveying means, for example, the belt or starwheel device that initially brought the container to the capping apparatus.
The containers capped by such a rotary capping apparatus must be subsequently unscrewed by hand to permit dispensing of the contents. Thus, the caps must be applied with sufficient torquing force so as not to leak during storage and transportation to the consumer, but may not be so tightly applied as to make it difficult for the consumer to remove the cap using only finger force. Consequently, the amount of torque applied must be within predetermined limits.
The prior art shows numerous patents in the field of capping devices for controlling the torque applied to such screw-on caps for containers. Most of the devices shown in the prior art use spring or air actuated friction slip clutches. In recent years, magnetic clutches or magnetic drives have also been frequently employed to control the torque applied to the caps.
Some examples of rotary capping devices in the prior art which utilize a disk clutch in the capping chuck are described in U.S. Pat. Nos. 4,558,554, 5,148,652 and 5,983,596. These disk clutches are comprised of a number of friction plates stacked together. The amount of torque applied on the caps is controlled by a mechanical adjustment of the pressure in the friction plates. Once the desired torque is applied, the friction clutch will slip and interrupt the connection with the actuating means. At this point the gripping means are gradually opened to disengage from the cap and to allow the next container to be fed into the device, and the application head is lifted away from the container to allow the next container to be fed into the device. The disk clutches can also be actuated by pressure from a compressed air source. These clutches are known as air clutches and permit more accurate control of the pressure on the friction plates through an air pressure regulator and an air pressure gauge. In such air clutches an air piston is carried in the underside of an air clutch hub between a pair of piston seals and a retaining ring. The air clutch mechanism senses the applied torque between the cap and neck of a container and will allow the cap tightening discs thereon to stop once the desired torque is reached. The air pressure regulator can vary the air pressure to the air clutch piston to change the tension on the friction plate assembly thereby varying the torque setting.
Some examples of the use of magnetic clutches in the prior art are described in U.S. Pat. Nos. 5,197,258 and 5,437,139. In these patents, a pair of axially aligned circular cylinders is provided. Each of the cylinders is provided with cavities containing magnets. The maximum torque provided by the clutch is controlled by the vertical distance between the two disks through removable spacer disks of varying thickness. By providing a greater number of spacer disks, finer adjustment in torque values can be achieved.
The cap gripping mechanisms of the prior art are indeed diverse. Perhaps the most common mechanism is a tapered insert inside an aperture for engagement with caps of different sizes as exemplified in U.S. Pat. No. 5,148,652. Another common device is the use of two or three gripping jaws as disclosed in U.S. Pat. Nos. 4,232,499 and 5,983,596. The capping chucks in these patents have retaining jaws that are adapted to receive and support a cap and to cooperate with an internal torque release lever and torsion spring arrangement operative to release the jaws from the cap after a predetermined rotational torque is applied between the cap and a container.
Still another cap gripping mechanism is disclosed in U.S. Pat. No. 5,459,975. The chuck disclosed in this patent has a plate that provides a seat for a flat elastomeric ring, which constrains the ring against radial expansion. The elastomeric ring defines an opening to accommodate the cap to be tested. The housing further accommodates a so-called pusher member, which normally engages the elastomeric ring. A cam applies a force to move the pusher member against the elastomeric ring and this force coacts with the constraining force of the annular plate to cause the elastomeric ring to expand inwardly into tight gripping engagement with a cap disposed within the elastomeric ring permitting torque to be applied to the cap by rotation of the chuck without deforming the cap.
Although the methods and apparatus for capping containers described hereinabove are effective, the capping devices of the prior art have inherent limitations, which require further improvement. Due to the difficulty in making adjustments to the torque exerted during the cap-tightening process, the prior art mechanisms for tightening caps onto containers have resulted in leaking containers requiring time consuming and expensive reprocessing. Also the mechanisms for gripping such screw-on caps frequently damage the caps due to the use of excessive and/or non-uniform gripping forces. If too much compression force is applied to the cap, it may be damaged or deformed resulting in faulty application of torque, or the cap may bind and not screw onto the container properly causing the containers to be rejected.
The cap gripping mechanisms of the prior art need improvement for the following additional reasons. Such cap gripping mechanisms of the prior art often employ gripping jaws, which are mechanically complex, expensive, difficult to adjust for individual cap sizes and shapes or which are custom made for each different cap size and shape. Such mechanically complex gripping mechanisms also introduce potential operator error into the capping process requiring complicated adjustments and resultant time losses during production set-up for different products. In addition, such mechanical gripping jaws require manual set-up and do not provide for computer-controlled adjustment to different cap sizes. Additionally, prior art capping devices have generally been configured such that when chuck jaws have to be repaired or replaced, either due to changes in the sizes of the caps and/or containers being processed or due to damage to the jaws in use, extensive delays are encountered while the capping apparatus is disassembled to allow the chucking jaws to be serviced.
Prior art cap gripping mechanisms that utilize a tapered aperture for engagement with caps depend on frictional engagement between the aperture and the contact area of the cap. It is well known that friction is an unstable parameter and that the friction coefficient varies significantly with ambient conditions and the shape of contact surfaces often causing slippage. This slippage is more likely to occur when there is a relatively small contact area between the cap and tapered aperture of the gripping device. Such slippage will cause rapid wear of the gripping device having a detrimental effect on gripping performance as well. In addition, the fixed size of such tapered-aperture gripping mechanisms does not allow for computer-programmable changeover for different cap sizes.
Prior art gripping mechanisms utilizing an elastomeric ring that expands inwardly into tight gripping engagement with the cap have the inherent disadvantage of wearing relatively quickly because the elastomeric ring deforms all of its volume and still has a limited contact area with the cap. Also, different cap sizes and shapes require manual change over to different tooling. In addition, such cap gripping mechanisms do not allow for computer-programmable adjustment for different cap sizes.
Prior art torquing mechanisms having a disk clutch in the chucking device have the disadvantage of not utilizing any feedback in compensating significant errors affecting the capping torque. Large variations in such error is due to friction fluctuation in clutch disks due to changes in ambient conditions, especially temperature rising during the slippage, and wearing of slipping surfaces. Any required changeover to different torque settings will require numerous set-up samples and many adjustments and may still result in unstable torque. In addition, the disk clutch type torque mechanism does not allow for computer-adjustable torque over a large torque range.
Other prior art torquing mechanisms utilizing magnetic clutches in the capping chuck have the disadvantage of lacking any feedback in compensating for significant error affecting the capping torque. In such torquing mechanisms any changeover to different torque requires manual exchange of so-called spacer disks for varying the magnetic force. In addition, such magnetic clutch torquing mechanisms do not provide for computer-controlled adjustment of torquing changes over the entire torquing range.
Accordingly, the present invention is a rotary capping apparatus and feedback control system for regulating the torque applied to screw-on type caps for industrial containers such as pharmaceutical containers. The present capping apparatus and feedback control system is integrated into a machine suitable for so-called clean room production, which provides for automated, sterile processing of such caps and containers. In the present invention such caps are gripped by an inflatable chucking device actuated by compressed air including an elastomeric insert that grasps the entire surface of the cap and not just a few contact points about a top edge of the cap as in prior art devices. Thus, in the present apparatus the pressure applied via the inflatable chucking device can be minimal. This significantly increases the life of the tooling and the stability of performance, reduces pressure on the periphery of the cap, and also prevents deformation of the cap.
The present capping apparatus also provides for positive gripping, that is, undesired slippage or slippage as a means of metering the torque is totally eliminated. The gripping force is sufficient to prevent any slippage between the cap and the inflatable chucking device. The minimum required gripping force can be varied for different caps and can be adjusted by a computer-programmable pressure regulator thereby providing programmable changeover for different applications. This eliminates operator involvement and associated human error and reduces production down time by allowing immediate changeover by selection of new parameters from a computer console. The gripping force is released by purging (or vacuuming for increased speed) the pressurized air from the inflatable elastomeric insert surrounding the cap.
The present invention is also able to control torque more accurately by the use of a closed loop feedback control system including a servomechanism to control the applied torque. In the present feedback system a comparison between the actual process condition and the desired condition is made. The difference between these two signals (i.e. the error) is fed into the control system, which uses this information to alter the output signal to attain the required torque value calculated as: Error signal=set pointxe2x88x92measured value. More specifically, in this application the actual torque being applied on the caps can be continuously fed back into the system for further action until the desired torque applied on the caps is reached. The present apparatus uses a proportional, integral and derivative known as a (PID) control system to control the applied torque for purposes of this invention. Such a PID control system consists of the following major components: a central processing unit (CPU), an input section, and output section, a power supply and a computer program.
The torque in the present capping apparatus is applied to the cap via a computer (CPU) controlled servomechanism. The servomechanism is engaged with the inflatable chucking device and executes closed loop PID control with position feedback, which results in precise torque application. Moreover, the value of the applied torque is adjustable from the computer console allowing for immediate changeover to different products, and eliminates any operator error associated with mechanical adjustments. The driver of the servomechanism is a servomotor. When the desired torque value is reached, the CPU immediately interrupts the PID controlling loop and removes voltage from the servomotor.
This system represents a significant improvement over the prior art capping devices described hereinabove wherein so-called open-loop control is used. In such devices no information is fed back to the system to determine whether the desired output was achieved and consequently a large error in the desired applied torque may result. Many outside influences affect the operation of such prior art capping devices. For example, the friction coefficient varies significantly with ambient conditions and shape of the cap engaging surfaces often causing slippage. Such slippage is more likely to occur due to a relatively small contact area between the cap and tapered aperture of the gripping chuck. Such slippage will often cause rapid wear of the gripping chuck and will generate heat. Both the resultant wear of the gripping chuck and the heat generated adversely impact the accuracy of the applied torque.
The present rotary capping apparatus also features automatic secondary height adjustment functions such that the machine will automatically set the vertical height of the cap dispensing mechanism based on a computer program for a specific product selected. This function is carried out manually in the prior art devices.
Other features and technical advantages of the present invention will become apparent from a study of the following description and the accompanying drawings.